U.S. patent application number 13/245243 was filed with the patent office on 2012-03-29 for hollow cylindrical thermal shield for a tubular cryogenically cooled superconducting magnet.
Invention is credited to Simon James Calvert.
Application Number | 20120075045 13/245243 |
Document ID | / |
Family ID | 43128083 |
Filed Date | 2012-03-29 |
United States Patent
Application |
20120075045 |
Kind Code |
A1 |
Calvert; Simon James |
March 29, 2012 |
HOLLOW CYLINDRICAL THERMAL SHIELD FOR A TUBULAR CRYOGENICALLY
COOLED SUPERCONDUCTING MAGNET
Abstract
A hollow cylindrical thermal shield for a tubular cryogenically
cooled superconducting magnet, has a first axis, an inner
cylindrical tube having an axis aligned with the first axis, an
outer cylindrical tube of greater diameter than the diameter of the
inner cylindrical tube, having an axis aligned with the first axis,
and annular end pieces, joining the inner cylindrical tube and the
outer cylindrical tube to form an enclosure. The hollow cylindrical
thermal shield further has a cylindrical stiffener, extending
axially at least part of the axial length of the inner cylindrical
tube, the stiffener being joined at intervals to the inner
cylindrical tube, thereby to improve the mechanical rigidity of the
inner cylindrical tube.
Inventors: |
Calvert; Simon James;
(Witney, GB) |
Family ID: |
43128083 |
Appl. No.: |
13/245243 |
Filed: |
September 26, 2011 |
Current U.S.
Class: |
335/216 |
Current CPC
Class: |
H01F 6/04 20130101; G01R
33/3804 20130101 |
Class at
Publication: |
335/216 |
International
Class: |
H01F 6/00 20060101
H01F006/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 28, 2010 |
GB |
1016290.7 |
Claims
1. A hollow cylindrical thermal shield for a tubular cryogenically
cooled superconducting magnet comprising an annular coil, having a
first axis and comprising: an inner cylindrical tube having an axis
aligned with the first axis; an outer cylindrical tube of greater
diameter than the diameter of the inner cylindrical tube, having an
axis aligned with the first axis; annular end pieces, joining the
inner cylindrical tube and the outer cylindrical tube to form an
enclosure; and the hollow cylindrical thermal shield further
comprising a cylindrical stiffener, extending axially at least part
of the axial length of the inner cylindrical tube, the stiffener
having a radially inner diameter greater than a radially outer
diameter of the annular coil and being joined at intervals to the
inner cylindrical tube, with the inner cylindrical tube lying
radially within the annular coil, while the stiffener lies radially
outside of the annular coil, thereby increasing mechanical rigidity
of the inner cylindrical tube.
2. A hollow cylindrical thermal shield according to claim 1 wherein
the cylindrical stiffener is joined to the inner cylindrical tube
by pillars placed at intervals.
3. A hollow cylindrical thermal shield according to claim 1 wherein
the cylindrical stiffener is joined to the inner cylindrical tube
by hollow through-bores placed at intervals.
4. A thermal shield according to claim 3 wherein the stiffener
extends between the annular end pieces, and is mechanically
attached thereto.
5. A thermal shield according to claim 3 wherein at least part of
the stiffener forms part of an annular end piece of the thermal
shield.
6. A hollow cylindrical thermal shield according to claim 1 wherein
the cylindrical stiffener is joined to the inner cylindrical tube
by one or more elongate, arcuate supports extending around a
circumference of the thermal shield.
7. A hollow cylindrical thermal shield according to claim 6 wherein
the elongate arcuate support(s) comprise several arcs spaced apart
in the circumferential direction.
8. A thermal shield according to claim 7 wherein at least part of
the stiffener forms part of an annular end piece of the thermal
shield.
9. A thermal shield according to claim 1 wherein the stiffener is
located adjacent to one of the annular end pieces, and is
mechanically attached thereto.
10. A hollow cylindrical thermal shield according to claim 1 having
an axial center, wherein at least one of the annular end pieces is
formed in three axially concentric pieces, located at least two
separate axial locations, such that radially inner and radially
outer pieces of the end piece are positioned to accommodate
respective inner and outer coils of a cryogenically cooled magnet,
while the radially intermediate piece is positioned axially nearer
an axial center than the respective radially inner and radially
outer pieces.
11. A thermal shield according to claim 10 wherein the three
axially concentric pieces are all planar, and are arranged
perpendicular to the first axis.
12. A thermal shield according to claim 10, wherein the radially
intermediate piece is frusto-conical, such that a radially outer
extremity of the radially intermediate piece is axially further
from the axial center than a radially inner extremity of the
radially intermediate piece.
13. A hollow cylindrical thermal shield for a tubular cryogenically
cooled superconducting magnet comprising adjacent annular coils,
having a first axis and comprising: an inner cylindrical tube
having an axis aligned with the first axis; an outer cylindrical
tube of greater diameter than the diameter of the inner cylindrical
tube, having an axis aligned with the first axis; annular end
pieces, joining the inner cylindrical tube and the outer
cylindrical tube to form an enclosure; and the hollow cylindrical
thermal shield further comprising a cylindrical support, extending
between adjacent coils, having an inner diameter less than the
outer diameter of at least one of the adjacent coils and having an
axial extent less than an axial spacing between the adjacent coils
and being joined at intervals to the inner cylindrical tube,
thereby increasing mechanical rigidity of the inner cylindrical
tube.
14. A hollow cylindrical thermal shield according to claim 13
wherein the cylindrical stiffener is joined to the inner
cylindrical tube by pillars placed at intervals.
15. A hollow cylindrical thermal shield according to claim 13
wherein the cylindrical stiffener is joined to the inner
cylindrical tube by hollow through-bores placed at intervals.
16. A hollow cylindrical thermal shield according to claim 13
wherein the cylindrical stiffener is joined to the inner
cylindrical tube by one or more elongate, arcuate supports
extending around a circumference of the thermal shield.
17. A hollow cylindrical thermal shield according to claim 16
wherein the elongate arcuate support(s) comprise a complete annular
support extending around the circumference of the thermal
shield.
18. A hollow cylindrical thermal shield according to claim 16
wherein the elongate arcuate support(s) comprise several arcs
spaced apart in the circumferential direction.
19. A hollow cylindrical thermal shield according to claim 16
wherein the elongate arcuate support(s) comprise several arcs which
overlap in the circumferential direction.
20. A hollow cylindrical thermal shield according to claim 13
having an axial center, wherein at least one of the annular end
pieces is formed in three axially concentric pieces, located at
least two separate axial locations, such that radially inner and
radially outer pieces of the end piece are positioned to
accommodate respective inner and outer coils of a cryogenically
cooled magnet, while the radially intermediate piece is positioned
axially nearer an axial center than the respective radially inner
and radially outer pieces.
21. A thermal shield according to claim 20 wherein the three
axially concentric pieces are all planar, and are arranged
perpendicular to the first axis.
22. A thermal shield according to claim 20, wherein the radially
intermediate piece is frusto-conical, such that a radially outer
extremity of the radially intermediate piece is axially further
from the axial center than a radially inner extremity of the
radially intermediate piece.
23. A magnet system comprising: a hollow cylindrical thermal shield
having a first axis; superconducting coils located within the
hollow cylindrical thermal shield; said hollow cylindrical thermal
shield comprising an inner cylindrical tube having an axis aligned
with the first axis, an outer cylindrical tube of greater diameter
than the diameter of the inner cylindrical tube, having an axis
aligned with the first axis, annular end pieces, joining the inner
cylindrical tube and the outer cylindrical tube to form an
enclosure; the hollow cylindrical thermal shield further comprising
a cylindrical stiffener, extending axially at least part of the
axial length of the inner cylindrical tube, the stiffener having a
radially inner diameter greater than a radially outer diameter of
the annular coil and being joined at intervals to the inner
cylindrical tube, with the inner cylindrical tube lying radially
within the annular coil, while the stiffener lies radially outside
of the annular coil, thereby increasing mechanical rigidity of the
inner cylindrical tube; a hollow cylindrical outer vacuum container
having an axial center and a second axis and enclosing the thermal
shield and the coils, the outer vacuum container comprising an
inner cylindrical tube having an axis aligned with the second axis,
an outer cylindrical tube of greater diameter than the diameter of
the inner cylindrical tube having an axis aligned with the second
axis, and annular end pieces, joining the inner cylindrical tube
and the outer cylindrical tube to form an enclosure; one annular
end piece of the outer vacuum container having a re-entrant
portion, causing a radially intermediate portion of the one annular
end piece to be axially closer to the axial center of the outer
vacuum container than radially inner and radially outer extremities
of the one annular end piece; and the outer vacuum chamber having a
re-entrant portion joined at intervals to the inner cylindrical
tube of the outer vacuum chamber, thereby increasing mechanical
rigidity of the inner cylindrical tube of the outer vacuum
chamber.
24. A magnet system according to claim 23 wherein the re-entrant
portion of the outer vacuum chamber is joined at intervals to the
inner cylindrical tube of the outer vacuum chamber by
through-bores.
25. A magnet system according to claim 23, comprising a cylindrical
former on which at least some of the coils are mounted.
26. A magnet system comprising: a hollow cylindrical thermal shield
having a first axis; superconducting coils located within the
hollow cylindrical thermal shield; an inner cylindrical tube having
an axis aligned with the first axis, an outer cylindrical tube of
greater diameter than the diameter of the inner cylindrical tube,
having an axis aligned with the first axis, annular end pieces,
joining the inner cylindrical tube and the outer cylindrical tube
to form an enclosure; the hollow cylindrical thermal shield further
comprising a cylindrical support, extending between adjacent coils,
having an inner diameter less than the outer diameter of at least
one of the adjacent coils and having an axial extent less than an
axial spacing between the adjacent coils and being joined at
intervals to the inner cylindrical tube, thereby increasing
mechanical rigidity of the inner cylindrical tube; a hollow
cylindrical outer vacuum container having an axial center and a
second axis and enclosing the thermal shield and the coils, the
outer vacuum container comprising an inner cylindrical tube having
an axis aligned with the second axis, an outer cylindrical tube of
greater diameter than the diameter of the inner cylindrical tube
having an axis aligned with the second axis, and annular end
pieces, joining the inner cylindrical tube and the outer
cylindrical tube to form an enclosure; one annular end piece of the
outer vacuum container having a re-entrant portion, causing a
radially intermediate portion of the one annular end piece to be
axially closer to the axial center of the outer vacuum container
than radially inner and radially outer extremities of the one
annular end piece; and the outer vacuum chamber having a re-entrant
portion joined at intervals to the inner cylindrical tube of the
outer vacuum chamber, thereby increasing mechanical rigidity of the
inner cylindrical tube of the outer vacuum chamber.
27. A magnet system according to claim 26 wherein the re-entrant
portion of the outer vacuum chamber is joined at intervals to the
inner cylindrical tube of the outer vacuum chamber by
through-bores.
28. A magnet system according to claim 26, comprising a cylindrical
former on which at least some of the coils are mounted.
29. A magnet system comprising: a hollow cylindrical thermal shield
having a first axis; superconducting coils located within the
hollow cylindrical thermal shield; said hollow cylindrical thermal
shield comprising an inner cylindrical tube having an axis aligned
with the first axis, an outer cylindrical tube of greater diameter
than the diameter of the inner cylindrical tube, having an axis
aligned with the first axis, annular end pieces, joining the inner
cylindrical tube and the outer cylindrical tube to form an
enclosure, and the hollow cylindrical thermal shield further
comprising a cylindrical stiffener, extending axially at least part
of the axial length of the inner cylindrical tube, the stiffener
having a radially inner diameter greater than a radially outer
diameter of the annular coil and being joined at intervals to the
inner cylindrical tube, with the inner cylindrical tube lying
radially within the annular coil, while the stiffener lies radially
outside of the annular coil, thereby increasing mechanical rigidity
of the inner cylindrical tube; and a hollow cylindrical outer
vacuum container having an axial center and a second axis and
enclosing the thermal shield and the coils, the outer vacuum
container comprising an inner cylindrical tube having an axis
aligned with the second axis, an outer cylindrical tube of greater
diameter than the diameter of the inner cylindrical tube having an
axis aligned with the second axis, and annul end pieces, joining
the inner cylindrical tube and the outer cylindrical tube to form
an enclosure; and a bellows joining the inner cylindrical tube to
an annular end piece.
30. A magnet system according to claim 29 wherein an annular end
piece of the outer vacuum container has a re-entrant portion, with
radially intermediate portion of the annular end piece joined to
the bellows being axially closer to the axial center of the outer
vacuum container than radially inner and radially outer extremities
of the annular end piece joined to the bellows.
31. A magnet system according to claim 29, comprising a cylindrical
former on which at least some of the coils are mounted.
32. A magnet system comprising: a hollow cylindrical thermal shield
having a first axis; superconducting coils located within the
hollow cylindrical thermal shield; the hollow cylindrical thermal
shield further comprising a cylindrical support, extending between
adjacent coils, having an inner diameter less than the outer
diameter of at least one of the adjacent coils and having an axial
extent less than an axial spacing between the adjacent coils and
being joined at intervals to the inner cylindrical tube, thereby
increasing mechanical rigidity of the inner cylindrical tube; and a
hollow cylindrical outer vacuum container having an axial center
and a second axis and enclosing the thermal shield and the coils,
the outer vacuum container comprising an inner cylindrical tube
having an axis aligned with the second axis, an outer cylindrical
tube of greater diameter than the diameter of the inner cylindrical
tube having an axis aligned with the second axis, and annul end
pieces, joining the inner cylindrical tube and the outer
cylindrical tube to form an enclosure; and a bellows joining the
inner cylindrical tube to an annular end piece.
33. A magnet system according to claim 32 wherein an annular end
piece of the outer vacuum container has a re-entrant portion, with
radially intermediate portion of the annular end piece joined to
the bellows being axially closer to the axial center of the outer
vacuum container than radially inner and radially outer extremities
of the annular end piece joined to the bellows.
34. A magnet system according to claim 32, comprising a cylindrical
former on which at least some of the coils are mounted.
35. A magnet system comprising: a hollow cylindrical thermal shield
having a first axis; superconducting coils located within the
hollow cylindrical thermal shield; said hollow cylindrical thermal
shield comprising an inner cylindrical tube having an axis aligned
with the first axis, an outer cylindrical tube of greater diameter
than the diameter of the inner cylindrical tube, having an axis
aligned with the first axis, annular end pieces, joining the inner
cylindrical tube and the outer cylindrical tube to form an
enclosure, and the hollow cylindrical thermal shield further
comprising a cylindrical stiffener, extending axially at least part
of the axial length of the inner cylindrical tube, the stiffener
having a radially inner diameter greater than a radially outer
diameter of the annular coil and being joined at intervals to the
inner cylindrical tube, with the inner cylindrical tube lying
radially within the annular coil, while the stiffener lies radially
outside of the annular coil, thereby increasing mechanical rigidity
of the inner cylindrical tube; a hollow cylindrical cryogen vessel
having an axial center a second axis and enclosing the coils,
comprising an inner cylindrical tube having an axis aligned with
the second axis, an outer cylindrical tube of greater diameter than
the diameter of the inner cylindrical tube and having an axis
aligned with the second axis, and annular end pieces, joining the
inner cylindrical tube and the outer cylindrical tube to form an
enclosure; and a bellows joining the inner cylindrical tube to an
annular end piece.
36. A magnet system according to claim 35 wherein an annular end
piece of the cryogen vessel has a re-entrant portion, with a
radially intermediate portion of the annular end piece joined to
the bellows being axially closer to the axial center of the outer
vacuum container than radially inner and radially outer extremities
of the annular end piece joined to the bellows.
37. A magnet system according to claim 35, comprising a cylindrical
former on which at least some of the coils are mounted.
38. A magnet system comprising: a hollow cylindrical thermal shield
having a first axis; superconducting coils located within the
hollow cylindrical thermal shield; the hollow cylindrical thermal
shield further comprising a cylindrical support, extending between
adjacent coils, having an inner diameter less than the outer
diameter of at least one of the adjacent coils and having an axial
extent less than an axial spacing between the adjacent coils and
being joined at intervals to the inner cylindrical tube, thereby
increasing mechanical rigidity of the inner cylindrical tube; a
hollow cylindrical cryogen vessel having an axial center a second
axis and enclosing the coils, comprising an inner cylindrical tube
having an axis aligned with the second axis, an outer cylindrical
tube of greater diameter than the diameter of the inner cylindrical
tube and having an axis aligned with the second axis, and annular
end pieces, joining the inner cylindrical tube and the outer
cylindrical tube to form an enclosure; and a bellows joining the
inner cylindrical tube to an annular end piece.
39. A magnet system according to claim 38 wherein an annular end
piece of the cryogen vessel has a re-entrant portion, with a
radially intermediate portion of the annular end piece joined to
the bellows being axially closer to the axial center of the outer
vacuum container than radially inner and radially outer extremities
of the annular end piece joined to the bellows.
40. A magnet system according to claim 38, comprising a cylindrical
former on which at least some of the coils are mounted.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a hollow cylindrical
thermal shield for a tubular cryogenically cooled superconducting
magnet, and particularly to such a thermal shield which is useful
in MRI (Magnetic Resonance Imaging) systems due to reduction in
gradient coil induced heating (GCIH) of cryogenically cooled
apparatus.
[0003] 2. Description of the Prior Art
[0004] Superconducting magnets for use in MRI systems are commonly
cylindrical in shape, and the present invention is directed to such
magnets. In an MRI system, a gradient coil assembly provides pulsed
magnetic fields to provide the required spatial encoding of the
imaging volume. Such time-variant magnetic fields will induce
heating into conductive materials in the vicinity.
[0005] FIG. 1 illustrates a typical arrangement of an MRI magnet
system. Coils 10 are wound onto a former (not shown) which is
placed within a cryogen vessel 12. The cryogen vessel is partially
filled with a liquid cryogen 15 such as helium to provide the
required cooling. A thermal radiation shield 16 surrounds the
cryogen vessel to shield it from radiated heat. The cryogen vessel
and the thermal shield are cooled by a cryogenic refrigerator 17.
The coils, former, cryogen vessel and thermal radiation shield are
surrounded by an outer vacuum chamber (OVC) 14. The volume between
the outer vacuum chamber 14 and the cryogen vessel 12 is evacuated.
Solid thermal insulation 18, such as aluminized polyester film, is
preferably placed in the space between the outer vacuum chamber 14
and the thermal radiation shield 16. Numerous other components,
such as mechanical support structures, are provided in a practical
MRI magnet system, but are not illustrated in the drawing for the
sake of clarity. In designing an MRI system, great effort is placed
on reducing thermal influx to the cryogen vessel; on maximizing the
diameter of the bore of the outer vacuum chamber; and on reducing
its length.
[0006] A cylindrical gradient coil assembly is typically placed
within the bore of the outer vacuum chamber.
[0007] The cryogen vessel, thermal radiation shield and outer
vacuum container are each typically hollow cylindrical enclosures,
each composed of an inner tube, an outer tube and two annular end
pieces joining the inner tube and the outer tube.
[0008] The inner tube of the thermal radiation shield is typically
of a highly electrically and thermally conductive material, such as
pure aluminum, and is about 6 mm thick. Such material is effective
at shielding the cryogen vessel from high-frequency (>100 Hz)
varying magnetic fields from the gradient coil assembly. Relatively
large eddy currents may be induced in the inner tube of the thermal
radiation shield due to the pulsing of a magnetic field by the
gradient coils. Such eddy currents cause heating of the thermal
radiation shield.
[0009] However, secondary and tertiary eddy currents remain a
problem. Although the cryogen vessel is not subjected to the
high-frequency varying magnetic fields of the gradient coils, the
magnetic pulsing of the gradient coils causes mechanical vibration
of the OVC and the thermal radiation shields. These vibrations,
within the magnetic field of the coils, cause induced eddy currents
in the material of the OVC and the thermal radiation shields. These
induced eddy currents in turn cause heating; and the magnetic
fields generated by the induced eddy currents induce further eddy
currents, and cause heating, in the cryogenically cooled components
such as coils 10 and cryogen vessel 12. All of such heating is
collectively known as gradient coil induced heating (GCIH).
[0010] The heating is particularly pronounced in cases where the
pulsing of the gradient coils is at a frequency near the resonant
frequencies of the inner tube of the OVC and the inner tube of the
thermal radiation shield. It is believed that the proximity of the
resonant frequencies is a feature of nested tubes of similar
dimensions, even where the tubes are of differing materials.
[0011] In magnet systems such as illustrated in FIG. 1, the coils
10 themselves are cooled by liquid cryogen 15 and will not be
heated by the GCIH. However, an increased boil-off of cryogen will
occur due to GCIH of the cryogen vessel and the coils and radiant
heating caused by GCIH of the thermal radiation shield.
[0012] Recent developments have led to magnets described as "low
cryogen inventory" or even "dry" magnets. In such designs, little
or no liquid cryogen is provided to cool the magnets. In "low
cryogen inventory" magnets, a relatively small volume of cryogen
circulates in thermal contact with the magnet coils, and is cooled
by a cryogenic refrigerator as it circulates. In a "dry" magnet, no
cryogen is provided, but a cryogenic refrigerator is thermally
linked to the magnet through a thermally conductive link such as a
copper or aluminum braid or laminate.
[0013] In "low cryogen inventory" or "dry" magnets, there is not a
large volume of cryogen to absorb heating of the cryogen vessel or
the shield due to GCIH. As a result, there is a risk that the coils
10 will heat, and quench, even in response to a relatively small
amount of heating. It is therefore particularly important to
minimize GCIH in "low cryogen inventory" or "dry" magnets. This may
be addressed by intercepting heat generated by GCIH, either in the
gradient coils, at the OVC inner tube, or at the thermal shield.
The present invention is particularly directed to intercepting the
majority of heat resulting from GCIH at the thermal radiation
shield.
[0014] Some attempts have already been made to address this
problem. In some arrangement (e.g. U.S. Pat. No. 7,514,928), the
cryogen vessel has been coated or lined with copper. This does not
prevent or reduce the magnitude of eddy currents in the cryogen
vessel, but reduces the resultant heating due to the reduced
electrical resistance of the cryogen vessel. This approach has been
found to have limited success, as the reduced resistance of the
cryogen vessel has been found to lead to increased eddy
currents.
[0015] The mechanical vibration of the inner tube of the thermal
shield has been addressed (e.g. U.S. Pat. No. 7,535,225) by bonding
patches of a high modulus material, such as carbon-fiber reinforced
plastic CFRP, onto the shield's inner tube. Such an approach has
been found effective to change the resonant frequency of the
shield's inner tube only if a significant radial thickness of
stiffening material is used. This results in an increase in the
diameter of the coils, and a great increase in wire cost, in order
to keep the bore of the OVC at the required diameter.
SUMMARY OF THE INVENTION
[0016] Problematic peaks in GCIH occur when the gradient coils are
pulsed at frequencies close to the resonant frequencies of both the
inner tubes of the thermal radiation shield and the OVC.
Problematic mechanical resonance of the inner tubes may be reduced
by separation of the resonant frequencies of inner tube of the OVC
and the inner tube of the thermal radiation shield. Furthermore,
the magnitude of resonance may be reduced by substantial stiffening
of the shield bore tube thereby to minimize the amplitude of
mechanical vibration, and so reduce the magnitude of any secondary
or tertiary eddy currents and heating induced in the thermal
radiation shield, the cryogen vessel or other cryogenically cooled
components.
[0017] The present invention accordingly provides a structure
having a stiffer (more mechanically rigid) inner tube of the
thermal radiation shield. The inner tube of the thermal radiation
tube may be effectively made heavier, yet without increasing the
required coil diameter. There need be no increase in wire cost, or
reduction in bore diameter of the thermal radiation shield.
[0018] The stiffened inner tube of the thermal radiation shield
provides substantial separation of resonant frequencies of the
inner tubes of the OVC and the thermal radiation shield. The
amplitude of mechanical vibration due to gradient coil pulsing is
reduced, leading in turn to reduced eddy currents in the cold
mass.
[0019] The present invention includes a hollow cylindrical thermal
radiation shield having an inner cylindrical tube, and a
cylindrical stiffener, extending axially at least part of the axial
length of the inner cylindrical tube. The stiffener is of greater
diameter than the inner cylindrical tube, and is joined at
intervals to the inner cylindrical tube, thereby to improve the
mechanical rigidity of the inner cylindrical tube.
[0020] Preferably, the inner cylindrical tube of the thermal
radiation shield is thinner than in a conventional arrangement,
allowing coil diameters to be reduced, saving wire cost; or the
bore diameter of the OVC may be increased.
[0021] According to an aspect of the invention, the cylindrical
stiffener is able to react much of the force on the inner tube and
end pieces of the thermal radiation shield, enabling the inner tube
itself to be of thinner material than is conventional. This in turn
may permit a reduction in the diameter of the magnet coils, and a
corresponding reduction in wire cost; or the bore diameter of the
OVC may be increased. Furthermore, the inner tube and end pieces of
the thermal radiation shield may be constructed of high purity
aluminum.
[0022] The present application may be applied to "low cryogen
inventory" or "dry" magnets, as well as to conventional "wet"
magnets in which the superconducting coils are cooled by partial
immersion in liquid cryogen.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 shows a cross-section of a conventional
superconducting magnet system for MRI.
[0024] FIG. 2 shows an axial part cross section of an embodiment of
the invention.
[0025] FIG. 3 shows a partial cut-away view of an embodiment of the
invention.
[0026] FIG. 4 shows a partial radial cross-section through the
structure of FIG. 3.
[0027] FIGS. 5A and 5B show comparative simplified partial cross
sections of a conventional OVC with a conventional thermal
radiation shield and a conventional OVC with a thermal radiation
shield according to an embodiment of the invention.
[0028] FIG. 6 shows an axial part cross-section of an embodiment of
the invention.
[0029] FIG. 7 shows an axial part cross-section of an embodiment of
the invention.
[0030] FIG. 8 shows a partial cross-section through the structure
of FIG. 7.
[0031] FIG. 8A shows a variant of the embodiment shown in FIG.
8.
[0032] FIGS. 9A-9C illustrate alternative arrangements for annular
end pieces of thermal radiation shields and OVCs.
[0033] FIG. 10 shows an enlargement of a part of FIG. 9C.
[0034] FIGS. 11-12 illustrate a detail of certain embodiments of
the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0035] FIG. 2 shows an embodiment of the present invention, as
applied to a "wet" magnet. Features corresponding to features of
FIG. 1 are identified by corresponding reference numerals. FIG. 2
represents a part-axial cross section. The cross-section
essentially has reflectional symmetry about axial centre line B-B
and the magnet system is essentially symmetrical about axis
A-A.
[0036] In the illustrated arrangement, coils 10a, 10b, 10c, 10d are
mounted on a former 22. As is well-known in the art, the former may
be made up of three parts: a central part 22a carrying central
coils 10b, 10c, 10d, and two end-parts 22b each carrying an end
coil 10a.
[0037] Active shield coils 10s, well known in themselves, are
arranged on a separate mechanical support 41 at a greater radius
about axis A-A than the central coils 10b, 10c, 10d. A cryogen
vessel 12 surrounds the coils and former, and retains a liquid
cryogen.
[0038] According to a feature of this embodiment of the invention,
the annular end piece 24 of the cryogen vessel 12 is made up of
three concentric pieces 24a, 24b, 24c. The annular end piece 24 of
the cryogen vessel has a re-entrant portion 25, such that a
radially intermediate piece 24b of the annular end piece 24 is
axially closer to the axial centre B-B of the cryogen vessel than
the radially inner 24a and radially outer 24c piece of the annular
end piece. Auxiliary equipment may be installed within the
re-entrant portion, if desired.
[0039] According to a feature of the invention, the thermal shield
further comprises a cylindrical stiffener 30, extending axially
part of the axial length of the inner cylindrical tube 32 of the
thermal radiation shield 16. In the illustrated embodiment the
stiffener 30 is at least partially accommodated within the
re-entrant portion 25, between the cryogen vessel 12 and the
thermal radiation shield 16. In the illustrated embodiment, the
stiffener 30 is welded 31 to the annular end piece 33 of the
thermal radiation shield 16, but any suitable method of fastening
may be used.
[0040] The stiffener 30 is joined at intervals to the inner
cylindrical tube 32 by pillars 34. In the illustrated embodiment,
pillars 34 are positioned at radial intervals around the
circumference of the inner tube 32. The pillars may all be arranged
at a same axial location, axially between end coils 10a and the
nearest central coil 10b. The pillars may be arranged at differing
axial positions, as limited by the axial positions of the coils
10a, 10b.
[0041] In the illustrated embodiment, the pillars 34 are welded 36
to the stiffener 30 and attached to the inner tube 32 of the
thermal radiation shield by a countersunk screw 38. However, any
appropriate method of fastening may be used.
[0042] Each of the pillars 34 passes through a cross-bore 40 in the
cryogen vessel 12. Each pillar 34 passes through a cross-bore 40 so
as to extend between the inner tube 32 of the thermal radiation
shield and the cylindrical stiffener 30. Each cross-bore 40 is a
tubular, preferably cylindrical, tube of internal diameter somewhat
larger than the diameter of the corresponding pillar 34. The
illustrated example shows the cross-bore tube welded into place in
the cryogen vessel. While any suitable method of attachment may be
used, welding may be preferred as it can be made leak-tight and
mechanically robust. A corresponding hole 42 must be provided in
the former 22 at each location where a cross-bore is provided, to
enable the cross-bore 40 to provide access between the inner tube
32 of the thermal radiation shield 16 and the stiffener 30.
[0043] The axial extremities of the inner tube 32 of the thermal
radiation shield 16 are the parts which are most affected by the
pulsed magnetic field of the gradient coils in operation. The axial
extremities are significantly stiffened by their mechanical linking
to the cylindrical stiffener 30. This results in reduced mechanical
vibration of the inner tube 32 in response to pulsed magnetic field
from the gradient coils, in turn leading to reduced GCIH heating of
the thermal radiation shield and reduced secondary eddy current
heating of the cryogen vessel 12 and the coils 10. The thermal
radiation shield is braced by the cylindrical stiffener, giving
high rotational stiffness to the joint between the end piece 33 and
the inner tube 32.
[0044] FIG. 3 illustrates a partial cut-away view of another
embodiment of the present invention. Features corresponding to
features shown in FIGS. 1, 2 carry corresponding reference
numerals.
[0045] In the embodiment of FIG. 3, the magnet coils 10a-10d are
constructed as a self-supporting assembly, for example a
resin-impregnated series of coils with axially oriented supporting
spacers 44 formed as an integrated monolithically impregnated
component. As such, a former such as shown in FIG. 2 is not
required. Furthermore, the magnet of FIG. 3 is a "dry" magnet. No
cryogen vessel is supplied, but the magnet coils 10 are cooled by
thermal conduction to a cryogenic refrigerator (not
illustrated).
[0046] In this embodiment, the cylindrical stiffener 30 extends the
full axial length of the magnet. It is attached to both annular end
pieces 33 of the thermal radiation shield 16. In the illustrated
embodiment, this attachment is achieved by a discontinuous welding
46, although any suitable method may be used. Openings 48 may be
provided in the cylindrical stiffener, to permit attachment of
shield coils (not illustrated) to the remainder of the magnet, by
any suitable means, and for attachment of mechanical supports to
the coils, preferably by mechanical attachment to spacers 44.
[0047] The stiffener 30 is joined at radial and axial intervals to
the inner cylindrical tube 32 by pillars 34. In the illustrated
embodiment, pillars 34 are positioned at radial intervals around
the circumference of the inner tube 32 at various axial positions.
The pillars are arranged at axial locations between coils 10, and
at radial circumferential positions between axially-oriented
spacers 44.
[0048] In the illustrated embodiment, the pillars 34 are attached
to the stiffener 30 and to the inner tube 32 of the thermal
radiation shield by countersunk screws 38. However, any appropriate
method of fastening may be used.
[0049] The inner tube 32 of the thermal radiation shield is
significantly strengthened by its mechanical linking to the
cylindrical stiffener 30. This results in reduced mechanical
vibration of the inner tube 32 in response to pulsed magnetic field
from the gradient coils, in turn leading to reduced GCIH and
reduced secondary and tertiary eddy current heating of the thermal
radiation shield. As the axial extremities are most susceptible to
secondary eddy current generation, it may be found sufficient to
mechanically link the stiffener 30 and the inner tube 32 only in
the regions of the axial extremities, near end coils 10a.
[0050] FIG. 4 shows a partial radial cross-section through the
structure of FIG. 3, at a position corresponding to axial position
IV-IV.
[0051] FIGS. 5A and 5B show simplified partial cross sections
comparing a conventional OVC 14 and thermal radiation shield 16
(FIG. 5A) with a cryogen vessel 14 and thermal radiation shield 16
provided with a stiffener 30 according to the present invention
(FIG. 5B). This diagram clearly illustrates that the stiffener of
the present invention does not reduce the available inner tube
diameter of the OVC. Indeed, use of stiffener 30 according to the
invention may allow a thinner inner tube 32 to be used increasing
the available inner tube diameter of the OVC.
[0052] FIG. 6 illustrates a partial axial cross-section of a
superconducting magnet assembly according to another embodiment of
the present invention. Again, only one axial extremity of the
assembly is illustrated, and the assembly is substantially
symmetrical about axis A-A.
[0053] In this embodiment, coils 10a, 10b are attached on their
radially outer surface to an external former 50. An intermediate
layer 52, for example of epoxy-impregnated fiberglass cloth, may be
provided between the coil 10 and the external former 50. The
external former may be a single tubular piece, for example of
fiberglass reinforced epoxy resin, or may be made up of several
pieces, as illustrated, which may be arranged to interlock by
suitable end-profiling.
[0054] In the embodiment of FIG. 6, the magnet coils 10 are
constructed as self-supporting resin-impregnated coils externally
attached to a cylindrical support 50. As such, the former of FIG. 2
is not required. Furthermore, the magnet of FIG. 6 is a "dry"
magnet. No cryogen vessel is supplied, but the magnet coils 10 are
cooled by thermal conduction to a cryogenic refrigerator (not
illustrated).
[0055] In this embodiment, the cylindrical stiffener 30 extends the
full axial length of the magnet. It is attached to both annular end
pieces 24 of the thermal radiation shield 16. In the illustrated
embodiment, this attachment is achieved by welding 46, although any
suitable method may be used.
[0056] The stiffener 30 is joined to the inner cylindrical tube 32
at radial intervals around the circumference of the inner tube 32
by pillars 34. The pillars are arranged at axial locations between
coils 10, and through holes 54 formed in the cylindrical support 50
between end coils 10a and adjacent central coils 10b. If required,
a further series of pillars may be provided at another axial
location, between adjacent coils.
[0057] In the illustrated embodiment, the pillars 34 are attached
to the inner tube 32 of the thermal radiation shield by countersunk
screws 38 and to the stiffener by welding 36. However, any
appropriate method of fastening may be used.
[0058] The inner tube 32 of the thermal radiation shield is
significantly strengthened by its mechanical linking to the
cylindrical stiffener 30. This results in reduced mechanical
vibration of the inner tube 32 in response to pulsed magnetic field
from the gradient coil 23, in turn leading to reduced GCIH and
reduced secondary eddy current heating of the thermal radiation
shield 16. As the axial extremities are most susceptible to
secondary eddy current generation, it may be found sufficient to
mechanically link the stiffener 30 and the inner tube 32 only in
the regions of the axial extremities, near the end coils 10a.
[0059] FIG. 7 shows another part-axial cross section of another
embodiment of the present invention. In this embodiment, multiple
short cylindrical stiffeners 30' are provided, and each have an
inner radius less than the outer radius of at least some of: the
end coils 10a and the central coils 10b, 10c.
[0060] In this embodiment, the coils 10 are again mounted on an
external support 50, in the manner discussed with reference to FIG.
6. However, in this case, the pillars 34 do not pass through the
external support. Between coils, for example between coils 10a and
10b, a cylindrical stiffener 30' is provided, having an inner
diameter less than the outer diameter of at least one of the
immediately adjacent coils. The cylindrical support is essentially
annular, having an axial extent less than the axial spacing between
the immediately adjacent coils. Pillars 34 are provided at radial
intervals around the circumference of the cylindrical support, and
are attached between the cylindrical stiffener 30' and the inner
tube 32 of the thermal radiation shield at radial intervals. In the
illustrated embodiment, the pillars are attached by countersunk
screws 38, but any suitable method for attachment may be used.
[0061] The inner tube 32 of the thermal radiation shield 16 is
significantly strengthened by its mechanical linking to the
cylindrical stiffeners 30'. This results in reduced mechanical
vibration of the inner tube 32 in response to pulsed magnetic field
from the gradient coil, in turn leading to reduced GCIH and reduced
secondary eddy current heating of the thermal radiation shield 16.
As the axial extremities of the inner tube 32 are most susceptible
to secondary eddy current generation, it may be found sufficient to
mechanically link the stiffeners 30' and the inner tube 32 only in
the regions of the axial extremities, near the outer coils 10a.
[0062] FIG. 8 shows a partial radial cross-section through line
VIII-VIII of the embodiment of FIG. 7.
[0063] In some embodiments of the invention, for example those as
illustrated in FIG. 7, some or all of the pillars 34 described,
mechanically linking the thermal shield to the cylindrical
stiffener(s) may be replaced by one or more elongate, arcuate
supports extending around a circumference of the thermal shield. In
an extreme case, the elongate arcuate support(s) may be complete
annular supports extending around the circumference of the thermal
shield.
[0064] FIG. 8A resembles FIG. 8, but illustrates an example of
elongate arcuate support 82, extending around a circumference of
the thermal shield 32, mechanically joining it to cylindrical
stiffener 30'. Elongate arcuate support 82 may be a complete
annular support extending around the circumference of the thermal
shield, or may be one of several arcs, which may overlap in the
circumferential direction, or may be spaced apart.
[0065] FIGS. 9A-9C illustrate alternative arrangements for annular
end pieces of thermal radiation shields and outer vacuum chambers
according to certain embodiments of the present invention.
[0066] In FIG. 9A, the thermal radiation shield 16 has a
conventional annular end-piece 33, which is essentially planar,
with some dishing at the outer radial extremity to form a convex
outer surface. The outer vacuum chamber 14 has a similarly shaped
annular end piece 47. This arrangement of thermal radiation shield
and outer vacuum chamber resembles a conventional magnet system.
The cylindrical stiffener 30 does not form a part of the outer wall
of the thermal radiation shield.
[0067] In FIGS. 9B, 9C, according to an optional feature of the
present invention, the cylindrical stiffener 30 forms part of the
outer wall of the thermal radiation shield 16, along at least part
of the axial length of the cylindrical stiffener 30.
[0068] In the arrangement of FIG. 9B, the thermal radiation shield
16 is made up of two hollow cylindrical thermal radiation shields
16a and 16b, linked by thermal radiation shields 16c shaped to
surround the supports 49 holding the shield coils 10s in place. The
annular thermal radiation shield 16a surrounding the inner coils
and end coils 10a, 10b, 10c, uses the cylindrical stiffener 30 as
its outer tube. The outer vacuum chamber 14 has shaped end pieces
56, each defining an annular cavity 58 extending around the
end-piece of the OVC. Alternatively, the cavity 58 may only be
part-annular, extending around part of the end-piece of the OVC. As
illustrated, the end-piece may have a curved profile in axial
cross-section, and may be formed by metal spinning.
[0069] In the arrangement of FIG. 9C, the end piece 64 of the
thermal radiation shield 16 is composed of three annular portions
64a, 64b, 64c, each of which is essentially planar. The structure
of the thermal radiation shield resembles that discussed with
reference to the cryogen vessel 12 in FIG. 2. Preferably, the three
portions 64a, 64b, 64c are all formed from a single planar piece of
material. The annular end piece 64 has a re-entrant portion, such
that radially intermediate portion 64b of the annular end piece is
axially closer to the axial centre of the outer vacuum container
than radially inner 64a and radially outer 64c portions of the
annular end piece. The radially intermediate portion 64b is
attached to radially inner 64a and radially outer 64c portions by
cylindrical wall portions 64d, 64e. Cylindrical wall portion 64d is
part of the cylindrical stiffener 30. The end-piece 64 thereby
defines an annular cavity 62 extending around the end-piece of the
thermal radiation shield, alternatively, the cavity 62 may only be
part-annular, extending around part of the end-piece of the thermal
radiation shield, by appropriate formation of the annular
end-piece. The end piece 60 of the OVC 14 is composed of three
annular portions 60a, 60b, 60c, each of which is essentially
planar. The structure of the OVC resembles that discussed with
reference to the cryogen vessel 12 in FIG. 2. Preferably, the three
portions 60a, 60b, 60c are all formed from a single planar piece of
material. The annular end piece 60 has a re-entrant portion, such
that radially intermediate portion 60b of the annular end piece is
axially closer to the axial centre of the outer vacuum container
than radially inner 60a and radially outer 60c portions of the
annular end piece. The radially intermediate portion is attached to
radially inner 60a and radially outer 60c portions by cylindrical
wall portions 60d, 60e. The end-piece 60 thereby defines an annular
cavity 66 extending around the end-piece of the OVC, alternatively,
the cavity 66 may only be part-annular, extending around part of
the end-piece of the OVC, by appropriate formation of the annular
end-piece.
[0070] Typically, in a completed MRI system, convex decorative
`looks` covers are placed over the OVC. Cavities 58, 66 define
volumes between the OVC and the looks covers which may be used to
accommodate auxiliary equipment, provided that it is tolerant of
the magnetic field in that volume.
[0071] FIG. 10 shows an enlargement of that part of FIG. 9C labeled
X. In particular, it illustrates an optional feature of the OVC.
Rather than having an inner tube 72 welded to an end piece 47; 56
using a fillet 70, as in FIGS. 9A, 9B the embodiment of FIG. 10 has
a thin inner tube 72 linked to the end piece 60a of the OVC by a
bellows 74. The presence of the bellows means that the inner tube
72 will not be subjected to any end loads, since atmospheric
pressure tending to displace the end piece 60a of the OVC will be
taken up by flexure of the end piece and displacement of the
bellows 74. Since the inner tube 72 then only has to resist
atmospheric pressure acting on its inner surface, it may be made
very thin. Use of a thin inner tube provides several advantages,
for example, a reduction in weight and material cost of the OVC,
opportunity to increase the bore diameter of the OVC for increased
patient comfort; or reduce the diameter of the coils for reduced
wire cost, or a combination of the two. The bellows 74 is
preferably a single convolution bellows.
[0072] Use of such a thin OVC inner tube, immune to end-loads,
allows its resonant frequency to become significantly separated
from that of the stiffened inner tube 32 of the thermal radiation
shield. The bore tube 32 of the thermal radiation shield may be
made much thinner than is conventional, as quench forces acting on
it are reacted by stiffened sections of the shield end and inner
tube.
[0073] The invention allows a large degree of `tuning` of resonant
behavior of shield structure, to ensure separation of the resonant
frequencies of the inner tubes of the thermal radiation shield, OVC
and cryogen vessel, if any.
[0074] While the present invention has been described with
reference to a limited number of example embodiments, various
modifications and variations will be apparent to those skilled in
the art. For example, while pillars 34 have been illustrated
joining the cylindrical stiffener 30 to the inner tube 32, any
other suitable mechanical joints may be employed. For example, in
each embodiment where a pillar 34 has been described, a hollow
through-bore, such as shown at 40 in FIG. 2, may be provided
instead. The use of such a through-bore may provide improved
mechanical rigidity as compared to a solid pillar, and may usefully
provide access for electrical conductors or other services between
coils of the magnet. The dynamic behavior of the inner tube 32 and
the stiffener 30 may be affected differently if hollow
through-bores are used instead of solid pillars, and this
differing, dynamic behavior may advantageously be used to ensure
separation of the resonant frequencies of the various concentric
tubes.
[0075] In an embodiment such as shown in FIG. 11, through-bores 80
are provided through the OVC, with through-bores 40 of larger
diameter being provided through the thermal radiation shield. An
enlargement of part of FIG. 11 is shown in FIG. 12. In the
illustrated embodiment, no cryogen vessel is provided, but a
similar embodiment could be constructed in which further
through-bores, concentric with through-bores 40, 80, are provided
linking inner and outer cylindrical walls of the cryogen vessel. By
providing such through-bores in the OVC, a useful route for passing
electrical cables and other services is provided, which may prove
particularly useful for embodiments on which auxiliary equipment is
provided within recess 25.
[0076] Such through-bores are preferably welded in position, as
schematically illustrated in FIGS. 11 and 12, for mechanical
strength and vacuum tightness.
[0077] Pillar 34 illustrated in FIG. 11, and the corresponding
feature in the embodiment of FIG. 9 may be replaced by one or more
elongate arcuate supports 82 as illustrated in FIG. 8A and as
described in the accompanying description, to provide increased
stiffness to the thermal radiation shield.
[0078] The provision of through-bores through the OVC may increase
its mechanical strength in regions susceptible to GCIH, and may
enable thinner materials to be used. Typically, it will be found
sufficient to provide through-bores 80 through the OVC 14 only at
some of the through-bores 40 of the thermal radiation shield 16.
The distribution of OVC through-bores 80 may be determined to
provide a required dynamic behavior, and to advantageously separate
the resonant frequency of the inner tube of the OVC away from the
resonant frequency of the inner tube of the thermal radiation
shield.
[0079] Although modifications and changes may be suggested by those
skilled in the art, it is the intention of the inventor to embody
within the patent warranted hereon all changes and modifications as
reasonably and properly come within the scope of his contribution
to the art.
* * * * *